test: revert debug program renames
DecoderAPI: _replay_gain() returns void

Let the function decoder_replay_gain() update
decoder_control::replay_gain_db instead of letting each decoder plugin
take care for that.
DecoderAPI: don't use replay_gain_get_real_mode()

replay_gain_get_real_mode() accesses dangerous globals.  Let's just
use the global variable replay_gain_mode, and assume "AUTO" means
"TRACK".  At this point, this is the best we can do, because neither
the decoder nor the player should know about the current replay gain
mode.  This is applied as a filter in the output thread.
filter/ReplayGain: add method _set_mode()

Push the new mode to the filter instead of accessing global variables
through replay_gain_get_real_mode().
Partition: new class, container for Playlist and PlayerControl

This is the beginning of multi-player support.  There will be support
for multiple Partition objects in one MPD process.